We had a batch of 60 S9s sent to one of our facilities brand new from Bitmain. At least 10 of them began to do this after just one month. After a lot of trial and error, it was found to be faulty fans. Bitmain admitted to us that they had some fans with issues causing this. It may be worth a shot swapping out the fans.

On another note, sometimes firmware updates can be iffy. Sometimes when not fully successful, the chip temps or the fan RPMs will not be read correctly. This will cause the miner to continually reboot. Reverting back to an older firmware and running it for 15 minutes can rule this out.